The proposed project involves an electrophysiological investigation of membrane properties of normal and mutant embryonic axolotl hearts (Ambystoma mexicanum). The cardiac-lethal mutation is due to the action of a single recessive gene, and is phenotypically expressed in the homozygous condition. The specific result of the mutation is a failure of the embryo to develop a heart beat. In this study I will utilize intracellular electrophysiological recordings in conjunction with ion substitutions and application of specific ion channel blockers to determine the current carrying ions for the rising phase, plateau and falling phase of the cardiac action potentials of both normal and mutant embryos. Also, the ionic basis of the resting membrane potential will be determined for each. Intercellular coupling and conduction of action potentials through the myocardial sheet will be analyzed with dual electrode recordings and current injection, and with the iontophoretic injection of the fluorescent dye Lucifer Yellow CH. Finally, the ultrastructure of intercellular junctions will be compared in the normal and mutant tissues. The normal/mutant axolotl preparation can be broadly used as a model "defective" excitable membrane, and is therefore of interest to all fields of bioelectrical research, particularly heart physiology and neurophysiology. The primary aim of this project is to determine the extent to which a single gene mutation can modify the electrical properties of an excitable membrane. In addition, this project will form the basis for a long-term investigation of heart membrane properties at different developmental stages, including the period prior to initiation of spontaneous contractions, and following ingrowth of modulatory neuronal inputs.